One of the most powerful pieces of knowledge that students can gain from the study of geology is an understanding of the immense scale of geologic time. In the author's introductory-level physical geology course at Northern Virginia Community College, they discuss geologic time about one-third of the way through the semester, after a thorough review of plate tectonics and the rock cycle. Because this subject is of foundational importance in all of his geology classes, he chose it as the focus for a Classroom Assessment Technique (CAT). Angelo and Cross (1993) present an extensive user's guide to many different CATs, but the key idea with any classroom assessment is to try to figure out if students are learning what they want them to learn. Based on the feedback an instructor gets from a CAT, he or she can decide whether to move on or re-examine a particular issue. In this article, the author offers his experiences in using CATs in his geology courses to gauge his students' grasp of course concepts.